1. Field of the Invention
The present invention relates to a polarizer rotating device for a multi polarized satellite signal and a satellite signal receiving apparatus having the same. More particularly, the present invention relates to a polarizer rotating device for a multi polarized satellite signal and a satellite signal receiving apparatus having the same with which it is possible to process a linearly polarized wave and a circularly polarized wave of a satellite signal.
2. Description of the Related Art
A reflector antenna has been widely used in satellite communication, a high-capacity radio communication, or the like. The reflector antenna is configured to focus a received signal into at least one focal point by using a principle of a reflecting telescope. In general, a horn antenna or a feedhorn may be provided at the focal point of the reflector antenna. Here, a parabolic antenna may be typically used as the reflector antenna.
The received signal is reflected from the reflector antenna to be transmitted to the feedhorn, and the feedhorn transmits the signal, which has been input to the feedhorn, to a low noise block down converter (LNB) through a waveguide. Then, the low noise block down converter converts the signal, which has received from the feedhorn, into a signal of an intermediate frequency band to transmit the converted signal to an external video playing media such as a TV set-top box. Here, the low noise block down converter is a device that corresponds to a first stage of receiving a signal and is referred to as a kind of electronic amplifier. Some noise is additionally introduced in the low noise block down converter, and the noise introduced in the low noise block down converter is amplified to be transmitted to the next stage. Such noise needs to be minimized in order to maintain an optimal system, and the low-noise block down converter is designed to have a minimum noise level in order to stabilize the entire satellite signal receiving system.
Meanwhile, a conventional low noise block down converter capable of processing a satellite signal of a specific band receives any one signal of a linearly polarized signal and a circularly polarized signal depending on polarization properties of signals received from a satellite.
In a satellite antenna provided on land, since the polarization property is determined depending on regions, a low-noise block down converter for a circularly polarized wave or a low-noise block down converter for a linearly polarized wave is used depending on the determined polarization property. Accordingly, the low-noise block down converter need not be replaced. Unfortunately, the polarization property of the satellite is changed along with the movement of a ship between nations or between continents such that the circularly polarized wave is changed to the linearly polarized wave or the linearly polarized wave is changed to the circularly polarized wave. Thus, a marine satellite antenna needs to selectively receive the linearly polarized wave or the circularly polarized wave. Disadvantageously, in order to selectively receive the linearly polarized wave or the circularly polarized wave, since it is necessary to replace the low-noise block down converter, there is a troublesome work.
In particular, since a marine satellite tracking antenna has a complicated device including a radome and is provided under antenna circumstances of shaking due to waves, if there is a lack of specialized knowledge about the assembly and disassembly of the marine antenna, it is difficult to manually replace a low noise block down converter for a circularly polarized wave and a low noise block down converter for a linearly polarized wave. In order to solve such a problem, there has been suggested an apparatus capable of receiving both the linearly polarized wave and the circularly polarized wave. However, such an apparatus has a large size unsuitable for a marine antenna or an antenna for a ship. Further, it is required that waveguides for individually receiving the linearly polarized wave and the circularly polarized wave are provided at the apparatus and a feedhorn antenna is moved to correspond to the individual waveguides. Thus, there is a demerit that the structure thereof is complicated.
In addition, when a conventional feeding system for a linearly polarized wave and a conventional feeding system for a circularly polarized wave are simply connected, it is difficult to commercialize the systems due to large loss caused by interference between the linearly polarized wave and the circularly polarized wave. When the feeding systems are separately attached, there is a problem that a manufacturing cost is excessively increased.
Furthermore, when a linearly polarized satellite signal is received, it is necessary to implement a function for automatically compensating for a skew angle in order to compensate for loss caused by a polarization angle caused between the linearly polarized satellite signal and a polarized wave received by the antenna. In other words, when the linearly polarized satellite signal is received, it is difficult to implement a function of controlling the skew angle by compensating for an error between a direction of the linearly polarized satellite signal and a polarization direction of the low noise block down converter for a linearly polarized wave and automatically aligning the low noise block down converter. Due to Faraday rotation caused when the linearly polarized signal transmitted from the satellite passes through the ionosphere, the skew angle is caused between the antenna at the transmission side and the low noise block down converter at the reception side. Since the skew angle causes polarization loss to attenuate the magnitude of the signal, it is necessary to compensate for the skew angle. The reason why the skew angle is caused is briefly explained below. Since all satellites exist above the equator of the earth and the earth is round, as the linearly polarized wave propagates toward the polar regions of the Earth, the linearly polarized wave is curved to cause the skew angle.
In order to receive a signal from the satellite that uses the linearly polarized wave depending on a position of the moving body such as a ship, it is required that the antenna is rotated by the skew angle to compensate for the skew angle. However, in such a method, since the antenna is rotated, there is a problem that the size of the antenna is increased, the manufacturing cost thereof is increased, and large power loss is caused.
For example, in Europe or Asia that uses the linearly polarized signal, in order to receive the linearly polarized satellite signal, there is an inconvenience that the antenna is rotated to compensate for the skew angle. Meanwhile, when the skew angle is not compensated, there is a problem that loss of the satellite signal is caused. In addition, since a moving body such as a ship, an airplane or a vehicle does not have a space enough to provide receiving apparatuses for respectively processing the linearly polarized wave and the circularly polarized wave, there is a great demand for a technology capable of receiving all the multi polarized waves by a single signal receiving apparatus and selectively receiving the circularly polarized wave or the linearly polarized wave while occupying a minimum operation space.